Study On The Preparation And Annexing Agent Of Electroless Ni-P Plating Deposited On AZ91D Magnesium Alloy

Magnesium alloys had became a kind of widely used engineering structure materialsbecause of its light weight, high specific strength and high specific stiffness, the excellentprocessing and casting properties. However,magnesium alloys are high reactive intrinsically,and have poor corrosion resistance and low hardness. These shortcomings become theimportant factors of restricting the further application of magnesium alloy severely.Electroless plating is a kind of surface treatment technology which can effectively improvethe surface corrosion and wear resistance of magnesium alloys. AZ91D magnesium alloys arethe main subjects studied in this paper.An optimum plating parameter has been obtained inmedium temperature and alkaline solution by the orthogonal experiment. Ni-P depositedcoatings are prepared under the optimum plating parameter. The corrosion resistance effect ofNi-P coatings is researched on different concentration of auxiliary complexing agent glycineand accelerator thioglycollic acid by dynamic potential polarization curves.The experiment results show: The capability of three experimental factors affectingcoating properties is solution temperature, concentration of corrosion inhibitor NH4HF2,pHvalue in turn. The optimum technology parameters are40℃、NH4HF214g/L、pH value7.5.Ni-P deposited coatings prepared under the optimum technology are nanometer crystallinestructure where some raised cellular objects have been distributed on the surface diffusely.Thickness of coatings is homogeneous and achieved5μm after plating30min. Coatings andmagnesium alloy matrix combined well and tightly. Phosphorus content in the coating is3.28mass%and belongs to low phosphorus coatings. Dynamic potential polarization curves showthat Ni-P platings can improve corrosion resistance of magnesium alloy matrix. The influence of glycine concentration changing on corrosion resistance of the coatingsis not remarkable. Corrosion potential and corrosion current density of Ni-P plating improvefirstly and decrease rearly with the increase of glycine concentration. They achieve to thehighest values commonly when the glycine concentration is8g/L. Due to the poorcomplexing capacity of glycine,phosphorous acid nickel precipitation produce in plating laterperiod when concentration is too high. Production of phosphorous acid nickel precipitationaffects the coating compactness and reduces corrosion resistance.Corrosion resistance of Ni-Pcoatings appears a trend of enhancing firstly and decreasing rearly with the increase ofthioglycolic acid concentration. Anode polarization curve of Ni-P plating shows a trend ofpassivation as the best addition amount of4g/L. S atoms in thioglycolic acid and P atoms inhypophosphorous acid root have a bonding effect and P-H key is weakened. Electrons enrichon the matrix surface after P-H key fractured and enhance reducing capacity ofhypophosphorous acid root. Nickel depositing rate is accelerated and corrosion resistance ofcoating is improved. High concentration of thioglycolic acid will adsorb on the metal surfaceand hinder electroless nickel plating process. Those reasons lead to reducing of coatingscorrosion resistance.